The life-cycle of Leishmania parasites within the sand fly vector includes the development of extracellular promastigotes from a noninfective, procyclic stage into an infective, metacyclic stage. These adaptations were explored in the context of the structure and function of the abundant surface lipophosphoglycan (LPG) on Leishmania donovani promastigotes. During metacyclogenesis, two important developmental modifications were observed. First, the size of the molecule is substantially increased due to a 2-fold increase in the number of phosphorylated disaccharide repeat units expressed. Second, there is a concomitant decrease in the presentation of terminally exposed sugars. The capping sugars on procyclic LPG were found to mediate procyclic attachment to the sand fly midgut, whereas these same sugars on metacyclic LPG failed to mediate metacyclic binding. And whereas intact metacyclic LPG did not inhibit procyclic attachment, depolymerized LPG inhibited as well as procyclic LPG, demonstrating that the ligands are normally buried. The exposure and subsequent masking of the terminal capping sugars explains the stage- specificity of promastigote attachment to and release from the vector midgut, which are key events in the development of transmissible infections in the fly. Phlebotomine vectors are in some instances able to transmit only certain species of Leishmania. Comparison of a large number of vector/parasite pairs revealed that species-specific differences in vectorial competence were in every case directly correlated with the ability of promastigotes to attach to the sand fly midgut, the variable outcomes of which were controlled by structural polymorphisms in the surface lipophosphoglycan (LPG) of the parasite. The data suggest that at least some phlebotomine vectors differ with respect to the parasite recognition sites which they express, and that midgut adhesion is a sufficiently critical component of vectorial competence as to provide the evolutionary drive for LPG structural polymorphisms. Mutants defective in expression of specific sugars involved in midgut attachment have been selected using lectins or antibodies which bind to the functional domains on LPG. LPG mutants have been obtained which fail to bind to the midgut or survive in the fly, and we are attempting to recover the defective genes by functional complementation.